Laser dye thermal transfer printing

ABSTRACT

A scanning laser beam heats selected regions of a dye donor ribbon to transfer dye to a receiver sheet to form an image therein. The beam scan rate and delay time between the start of successive pulses is set so that adjacent heated pixel regions overlap and so that a significant amount of residual heat from a first printed pixel is still present when heat is applied to the next adjacent pixel. The use of this residual heat improves the printing efficiency. An elliptical beam may have its major axis in the scan direction to facilitate overlap. Printing may be effected to a non-square grid with more information, or a repetition of information, a set number of times in the scan direction. The power applied to a pixel may be adjusted depending on the darkness of previous adjacent printed pixels.

BACKGROUND OF THE INVENTION

The present invention relates to a method of operating a laser source toeffect dye thermal transfer printing. There are three main types of dyethermal transfer printing methods, in which dye is transferred bymelting, diffusion and sublimation respectively.

In the diffusion method, a dye donor ribbon and a dye receiver ribbon,comprising a dye layer and a receiver layer, respectively, on asupporting substrate, are held in contact with one another, and alocalized source of energy is used to heat selected pixel regions of thedye layer to cause dye in those regions to become thermally mobile anddiffuse into the adjacent receiver layer to produce a pattern of printedpixels therein. A desired print may be produced by heating anappropriate selection of pixel regions in the dye layer. By applyingmore or less energy to a pixel region of the dye layer, more or less dyeis transferred to the receiver layer, and so darker or lighter printedpixels are produced. This allows for continuous tone printing.

Laser sources are often selected as the energy source, because they canprovide an intense, highly directional and controllable output. Whenlaser sources are selected, laser light absorbing material is normallyprovided in the dye ribbon, either as a separate layer or dispersedwithin the dye layer, to convert the laser energy to heat.

Typically, the output of a laser source is scanned across the donorribbon at a set speed, and the laser source output is pulsed on and off.A heated pixel region is produced in the dye layer whenever the outputis pulsed on, and the darkness of a printed pixel depends upon theamount of dye transferred to it from the corresponding heated pixelregion in the dye layer, which in turn, depends upon the power andlength of the laser pulse applied to that pixel region. The spacingbetween adjacent printed pixels depends upon the scan rate of the laseroutput across the dye layer and on the time between the start of thepulses producing the printed pixels.

A region of darkness may be produced in a print by printing a line ofadjacent dark pixels, and it is known to select the scan rate and laserpulse rate so that each printed pixel partly overlaps those adjacent itin the scan direction. This ensures that the printed dark region is ofconstant optical density, and compensates for the fact that each printedpixel is slightly darker at its centre than at its periphery due to thegaussian cross-sectional profile of a typical laser output. The overlapgenerally coincides with the half width points of the laser beam profile(measured at l/e of maximum profile intensity) when projected onto theprinted pixels.

SUMMARY OF THE INVENTION

The present invention relates to the more efficient operation of a lasersource in dye thermal transfer printing, and recognizes the advantagesof optimized modulation and control of laser pulse times and overlap, incontrast to the prior art, which does not.

From a first aspect, the invention provides a method of laser dyethermal transfer printing in which the output of a pulsed laser sourceis scanned across a dye donor element to heat selected pixel regions ofthe donor element to effect transfer of dye to a dye receiving element,wherein the scan rate of the output across the donor element and thedelay time between the start of successive laser pulses which apply heatto respective adjacent heated pixel regions are set such that theadjacent heated pixel regions overlap in the scan direction, and whereinthe delay time is further set such that a significant amount of residualheat from a first heated pixel region, which produces a printed pixel ofthe darkest shade, is still present in that region at the time ofapplication of heat to the adjacent heated pixel region.

The term "adjacent heated pixel regions" should be taken to define apair of heated pixel regions which are at the minimum spacing for theparticular scan rate and delay time used in the method, and should notbe taken to cover two heated regions what are produced one after theother but are separated by one or more pixel region widths, e.g. whenprinting an image having a dark pixel then a white region and thenanother dark pixel. In this latter case there could be no overlap orthermal interaction between the dark pixel regions.

The present invention provides a more efficient way of printing ascompared with the prior art.

In accordance with the present invention, at least where a first pixelregion is printed at the darkest shade by a full power pulse, there isstill a significant amount of heat in that pixel region when an adjacentoverlapping region is heated. Thus, rather than being wasted, thisresidual heat is utilised to enhance the heating effect in the adjacentpixel region.

The present invention provides a thermal interaction between adjacentheated pixel regions, at least where one is heated by the maximum laseroutput, and so enables the energy applied by a laser source to be usedmore efficiently, so that a print area of a set optical density can beproduced from a lower laser energy, as compared with the prior art.

The time delay is preferably set such that a thermal interaction existseven when the first pixel region is heated to less than the maximumextent, i.e. to produce a pixel of intermediate shade. As discussedabove, where there is a lightly colored pixel or pixels, or an unprintedregion, there will be no significant thermal interaction at all. In thepresently preferred system, it is estimated that significant thermalinteraction is obtained from a first pixel region printed to about 30%,of the maximum optical density.

It is preferred for the delay time to be about 1,000 microseconds orless, as the invention has been found to work particularly well with thedelay time below or at about this value.

It is further preferred for the delay time to be less than or equal toabout 10 microseconds, and, indeed it is preferred for the delay time tobe minimized, and to tend to zero. The preferred scan rate is from about0.1 m/s to about 100 m/s and the preferred laser power density is fromabout 10⁷ to 10¹⁰ Wm⁻².

It is also preferred for adjacent heated pixel regions to overlap by anamount greater than the laser cross-sectional profile half width pointoverlap used in the prior art and preferably to overlap by an amountabout 10% or more greater than the prior art overlap. By having agreater overlap, more residual heat from an adjacent heated pixel regionwill be available, thus increasing efficiency still further. The scanspeed of the laser output may therefore be set relative to the heatingdelay time to allow for a more substantial overlap of adjacent heatedpixel regions than is known in the prior art. The greater the overlap,the greater the interaction.

Having the pixel regions closer together implies that more pixels needto be printed in the present method than in the prior art methods toprint a dark area of a given size. This might be thought to be adisadvantage in terms of energy efficiency, but it has been found thatthe energy saving produced by the thermal interaction of the pixels morethan compensates for the need to print more pixels.

A laser source may be operated in the pulsed mode to print a region ofconstant optical density by successively pulsing the laser source on andoff to produce a line of overlapping pixels. Of course, to produce adesired print image, areas of different optical densities are needed,and so the laser energy applied to different pixel regions needs to bemodulated. This may be achieved by varying the power or length of thelaser pulse.

The laser source could be operated in a continuous mode instead of apulsed mode, in which case a line of constant optical density may beproduced by scanning a continuous beam across the dye layer. Thiscontinuous scanning corresponds to the limit of decreasing both theheating delay time and the pixel spacing to a minimum in effect to zero.However, modulation is required to print information, e.g. images, andcontinuous scanning alone will not provide this.

Thus, viewed from another aspect, the present invention provides amethod of laser dye thermal transfer printing, in which the output of alaser source is scanned across a dye donor element to effect transfer ofdye to a dye receiving element, wherein the laser source is continuouslyon when producing regions of darkest shade, and wherein the beam ismodulated to produce regions of lighter shade. This modulation may beachieved by varying the power of the beam and/or by rapidly modulatingthe laser source on and off, as it is scanned across the pixel regionsproducing the lighter shades, so that heating takes place only for adesired fraction of the time taken for the beam to cross these pixelregions. Preferred scan rates are 0.1 to 100 ms⁻¹.

Any suitable laser source may be used, such as an Nd:YAG laser or alaser diode, which can be operated in either the pulsed or continuousmode.

Although the invention provides thermal interaction in the scandirection, the time between the printing of successive scan lines willbe much greater than the time for each heated pixel region to coolfully. Therefore, no thermal interaction exists between heated pixelregions of, for example, a dye sheet which are adjacent one another inthe feed direction of the dye sheet, perpendicular to the scandirection. As there is no thermal interaction, there is no advantage inhaving an overlap between adjacent heated pixel regions in the feeddirection which is greater than that known from the prior art, and sothe pixel regions in the scan direction will generally be closer to oneanother than in the feed direction. Accordingly, a pulsed laser willgenerally print information as a non-square pixel data grid, with morepixel information being provided in the scan direction than the feeddirection. Therefore, to produce an image with data distribution in asquare grid, i.e. to provide equal resolution in both the scan and feeddirections, data in the scan direction could be printed repeatedly ntimes, where nx=y, x being the pixel spacing in the scan direction and ythe spacing in the feed direction.

Thus, the invention extends to a method of laser dye thermal transferprinting in which printing is effected to a non-square grid, with morepixel information being provided in the laser scan direction than adirection perpendicular thereto. The laser addressed locations may forma rectangular grid, and a square aspect ratio may be provided byrepeatedly printing each piece of data information a set number of timesin the laser scan direction.

Also, if a laser source is arranged to produce, for example, anelongate, e.g. elliptical, scanning spot, then it is preferred for thespot to be arranged with its long axis in the scan direction. Thisallows for a greater overlap between adjacent heated pixel regions, andalso allows a square data grid to be used for the print information.This may be particularly applicable to the use of laser diodes where theuncorrected beam is elliptical.

Thus, the invention also extends to a method of laser dye thermaltransfer printing, in which the laser spot scanning across the dye donorelement is elongate, e.g. elliptical, in cross-section, and in which thelong axis of the spot is arranged to lie in the direction of laser scan.Preferably, data is printed to the donor element using a square grid.

In a preferred embodiment, the amount of heat applied to a pixel regionto produce a printed pixel of a set tone is varied in dependence on thetone of one or more consecutively adjacent previously printed pixels inthe scan line. This takes into account the fact that the amount of dyetransferred depends not only upon the heat applied directly to a pixelregion but also upon the amount of residual heat in adjacent pixelregions. Thus, any residual heat from one or more adjacent heated pixelregions, and the heat applied directly to a pixel region, together, setthe darkness of a printed pixel, and, by making corrections for thevariations in the residual heat available, undesired tonal features areavoided in the finished print. This may be most noticeable when an imagehas a sharp transition in shade, e.g. a dark edge, where the thermalinteraction may result in a gradual transition of shade, i.e. a blurrededge. By compensating for the heat applied to a pixel region independence on the thermal history of the donor element, this blurringmay be avoided.

For example, the energy applied to a pixel region may be increased, ifone or more of the previously printed adjacent pixels is lighter in tonethan the pixel to be printed, and the amount of increase may depend onthe difference in tone of the previously printed pixels.

Thus, if a few pixels in a scan line are white and the next four areblack and of the same optical density, then the heated pixel regionproducing the first black pixel should receive more energy than thepixel region producing the second black pixel, which, in turn, shouldreceive more energy than the regions producing the third and fourthblack pixels. These third and fourth regions may be heated with a basicuncompensated energy based on the desired tone. The first compensatedregion may have a 100 percent increase in the basic energy, the secondheated region may have a 50 percent increase.

The feature of residual heat compensation is, in itself, significantand, viewed from a further aspect, the present invention provides amethod of laser dye thermal transfer printing in which the output of alaser source is scanned across a dye donor element to heat selectedpixel regions of the donor element to effect transfer of dye to a dyereceiving element to produce printed pixels of varying shade therein,wherein the energy applied to a pixel region of the donor element toproduce a printed pixel of a set shade is varied in dependence on theshade of one or more previously printed successively adjacent pixels inthe scan line.

A microprocessor may be used to control the printing apparatus andeffect the above printing methods.

BRIEF DESCRIPTION OF DRAWINGS

Embodiments of the present invention will now be described, by way ofexample only, with reference to the accompanying drawings, in which:

FIG. 1 is a schematic view of a printing system operable in accordancewith embodiments of the present invention;

FIG. 2 is a schematic diagram showing the overlap of a pair of adjacentpixel regions in a dye ribbon; and

FIG. 3 is a graph of pulse delay time against optical density.

DETAILED DESCRIPTION

Referring to FIG. 1, a printing system 1 comprises a dye receiver sheet2 mounted on a rotating drum 3, and a dye donor ribbon 4 held in tensionagainst the receiver sheet 2 by, for example, a pair of tensioningrollers (not shown). The output 5 from a laser source 6 is scannedacross the dye ribbon 4 by a rotating polygon mirror 7. A flat fieldlens 8 is provided between the polygon 7 and dye ribbon 4 to modify thelaser output 5 to scan in a flat focal plane rather than a curved one.

In one embodiment, the laser source 6 is modulated to produce an output5 which is pulsed on and off. As the output 5 is scanned across thedonor ribbon 4, selected pixel regions are heated by pulsing on thelaser output 5 at selected points in the scan. Dye transfers from theseheated regions to the receiver layer to produce corresponding printedpixels therein. Thus, a print image is producedpixel-line-by-pixel-line, as the drum 3 holding the receiver sheet 2rotates in synchronism with movement of the dye donor ribbon 4.

In accordance with a first embodiment of the present invention, whenproducing a region of constant high optical density, the time betweenthe start of successive output pulses from the laser source 5 is lessthan 1000 μs, and the scan rate is set in dependence on this pulse rateand the laser beam width to ensure that adjacent heated pixel regions ofthe dye ribbon overlap one another by an amount greater than thatcorresponding to the overlap at the half-width points of the laseroutput 5 cross-sectional profile. This is illustrated in FIG. 2. As anexample, the time between pulses could be 5 μs, the beam width could be20 μm, and the scan speed could be 1 ms⁻¹.

A microprocessor may be used to control the operation of the apparatus.

By using a pulse time of less than 1000 μs, a thermal interaction isproduced between adjacent heated pixel regions, so that residual heatremaining after a pixel region is heated is utilised in the subsequentheating of an adjacent pixel region. This reduces the energy required toproduce a set optical density, as compared with the prior art. By havinga large pixel overlap, this thermal interaction is increased.

As there is this thermal interaction, the energy applied to a pixelregion of the donor ribbon 4, to produce a pixel in the receiver sheet 2of a certain dye density, is modified to take account of the thermalhistory of the donor ribbon. This is because the residual heat from theprinting of one or more previously printed pixels together with a basicamount of heat directly applied to a pixel region for producing a setpixel tone, make the shade of the printed pixel darker than the settone, and uncompensated variations due to the variation in the residualheat contribution can produce undesirable print results. This may bemost noticeable when a printed image makes a sharp transition in shade,e.g. has a dark edge, as the thermal interaction results in the gradualtransition of shade, i.e. a blurred edge.

To prevent this, the energy applied to a heated pixel region to print apixel of a set tone is corrected to take account of any residual heatcontribution from the printing of a previous few adjacent pixels in thescan line. This compensation depends on the contrast of the grey levelsof, for example, the previous three consecutive adjacent pixels, so thatif the first few pixels in a scan line are white and the next four areblack and of the same optical density, then the heated pixel regionproducing the first black pixel must receive more thermal energy thanthe second, which, in turn requires more thermal energy than the thirdand fourth of the heated pixel regions forming the black pixels. Thethird and fourth pixel regions can be heated with their normaluncorrected thermal energy.

The amount of energy correction may be about 100 percent of theuncorrected thermal energy for the first pixel, and about 50 percent forthe second pixel.

Instead of pulsing the laser output 5, the output 5 could comprise acontinuous beam, which, when printing a region of high optical density,corresponds to the limit of a zero heating delay time and a completeoverlap and merging of the separate pixel regions. In order to produceregions of varying optical density, the laser output power may bemodulated, or may be rapidly pulsed on and off, so that the output 6 ison only for a desired fraction of the time taken for the beam to crossthat region.

An experiment was conducted to illustrate the advantages of reduceddelay time between pulses:

Dye sheets were prepared by gravure coating 23 μm S grade Melinex (ICI)with the following solutions to give cyan dye coat. The solution wascoated to give a dry coat weight of approximately 1 μm.

    ______________________________________            Cyan    ______________________________________            C1           0.865 kg            C2           1.298 kg            EC200        1.622 kg            EC10         0.541 kg            IRA          0.571 kg            THF          50 liters    ______________________________________

The following abbreviations for the dyes and binders, are used:

    ______________________________________    C1        3-acetylamino-4-(3-cyano-5-phenylazothiophenyl-              2-ylazo)-N,N-diethyaniline    C2        C1 solvent blue 63    PVB       Poly vinyl butyral BX1 from Sekisui    ECT       Ethyl Cellulose T10 from Hercules    S101743   Hexadeca-b-thionaphthalene Copper (II)              phthalocyanine    ______________________________________

Receiver sheets were prepared by coating, via the bead method, 125 μm Ograde melinex (ICI) with the following formulation from a solution in a50/50 mixture of toluene and MEK to give a dry coat weight of 3-4 μm:

    ______________________________________    Vylon 200             30     parts    cymel 303  6    parts    Vylon 103             70     parts    Tegomer HSi2210                                        0.7  parts    Ketjenflex MH             7.5    parts    Tinuvin 900                                        1    part    R4046    0.4    parts    ______________________________________

Vylon 200 and 203 are soluble polyesters with high dye affinity, fromToyobo. Tinuvin 900 is a UV absorber from Ciba Giegy. Ketjenflex MH is acrosslinking agent from Akzo. Cymel 303 is a hexamethyoxymethylmelamineoligomeric crosslinking agent from American Cyanamid. Tegomer HSi2210 isa bis-hydroxy alkylpolydimethylsiloxane releasing agent from THGoldschmidt and R4046 is an amine blocked para toluene sulphonic acidcatalyst.

The dye sheet/receiver sheet assembly was supported by a platen andbrought into contact by tensioning the dye sheet against the platen. Thetension on the dye sheet was just enough to provide the intimate contactbetween the dye sheet and the receiver sheet necessary for dye transfer.The laser used was a Spectra Diode Lab 150 mW semiconductor laseroperating at 820 nm wavelength. The laser power delivered to the dyesheet/receiver sheet assembly was about 100 mW and the beam spot sizewas about 38 μm. The scanner was a General Scanning galvanometer scannerhaving a 80 Hz bandwidth.

The laser on time and pixel spacings were kept constant at about 50 sand 20×20 μm, respectively. The delay time between pulses was thenvaried (as was the scan speed to keep the pixel spacing constant), andthe corresponding optical density of a uniform printed block was thenmeasured using a Sakura densitometer.

The results are indicated in FIG. 3, and shows that as the thermalinteraction increases, the energy efficiency increases. The mainincrease is at about 1000 μs.

We claim:
 1. A method of laser dye thermal transfer printing, comprisingthe steps of:(a) providing a modulated laser source; (b) selecting ascan rate and a delay time between starting points of successive laserpulses of a modulated laser output; and (c) scanning the modulated laseroutput across a dye donor element, thereby sequentially heating aplurality of selected pixel regions of the dye donor element andtransferring a dye from the dye donor element to a receiver elementdisposed adjacent the dye donor element, wherein the scan rate and thedelay time are selected to create adjacent heated pixel regions whichoverlap in a scan direction and by an amount greater than an overlap athalf-width points of a laser output cross-sectional profile.
 2. Themethod of claim 1, wherein step (b) comprises selecting the delay timeof about 1,000 microseconds or less.
 3. The method of claim 1, whereinstep (b) comprises selecting the delay time of about 10 microseconds orless.
 4. The method of claim 1, wherein step (b) comprises selecting thescan rate from about 0.1 m/s to about 100 m/s.
 5. The method of claim 1further comprising the step of selecting a laser power density fromabout 10⁷ to 10¹⁰ Wm⁻².
 6. The method of claim 1, wherein step (c)comprises scanning the modulated laser output so as to print data in thescan direction repeatedly n times, where nx=y, x is a pixel spacing inthe scan direction and y is a spacing in a direction perpendicular tothe scan direction.
 7. The method of claim 1, wherein step (c) comprisesscanning the modulated laser output across a dye donor element with anelongate scanning spot having a long axis in the scan direction.
 8. Themethod of claim 1 further comprising the step of selecting an amount ofheat applied to a pixel region to produce a printed pixel of a set tonebased on a tone of one or more previously printed adjacent pixelregions.
 9. The method of claim 8, wherein the step of selecting anamount of heat comprises increasing the amount of heat applied to apixel region if one or more of the previously printed adjacent pixelregions is lighter in tone than the pixel region to be printed.
 10. Themethod of claim 9, wherein the step of increasing the amount of heatcomprises increasing the heat by an amount based on a difference in toneof the previously printed pixel regions and the pixel region to beprinted.
 11. The method of claim 1, wherein step (b) comprises selectingthe scan rate and the delay time which create an overlap of 10% or moregreater than the overlap at half-width points of a laser outputcross-sectional profile.
 12. The method of claim 1, further comprisingthe step of providing a laser source which is continuously on whenproducing a plurality of regions of a darker shade on the receiverelement, and providing a laser source that is modulated when producing aplurality of regions of a lighter shade on the receiver element.
 13. Themethod of claim 1, wherein step (c) comprises scanning the modulatedlaser output so as to provide more pixel information per unit length inthe scan direction than in a direction perpendicular to the scandirection.
 14. The method of claim 13, wherein each piece of pixelinformation is repeatedly printed a set number of times in the scandirection.
 15. A method of laser dye thermal transfer printing,comprising the steps of:(a) providing a laser source to generate a laseroutput; (b) selecting an energy level for the laser output to create adesired tone on a receiver element; and (c) scanning the laser outputacross a dye donor element, thereby sequentially heating a plurality ofselected pixel regions of the dye donor element, transferring a dye fromthe dye donor element to the receiver element, and providing printedpixel regions with a varying shade on the receiver element; wherein theenergy level is selected based on a tone of one or more previouslyprinted adjacent pixel regions.